The invention relates to a method and apparatus for filtering an analog electrical signal utilizing a digital type filter.
The finite impulse response (FIR) filter is well known to the art. For example, a typical FIR filter can include a charge-coupled-delay having a number N of serially arranged registers for holding digital or analog signals. At a predetermined clocking rate, all the data held by the charged-coupled-delay is shifted to the next holding register as new data fills the first register and old data is lost from the Nth register. The output of each register is weighted and summed. The value of the weights determine the response of the filter stop band and the register length or delay time determines the filter bandwidth.
Charge-coupled-delay lines do not have the speed required for most filter applications. Therefore, digital circuits using TTL or COS/MOS circuits generally are used.
An FIR digital filter can be implemented using discrete digital circuits or a microprocessor. The conventional approach using a digital circuits or a microprocessor is to first convert the analog signal to a digital word by an analog-to-digital (A/D) converter. Each word is stored, weighted, and summed in the processor to form the FIR filter. Since multiplications are required to provide weights for the stored numbers, many milliseconds of computer time are required. Depending on the filter specification, there may be more time efficient filters which can be implemented than an FIR filter, but any known filter requires complex addition or multiplication. Typical examples of such known digital filtering devices are described in U.S. Pat. No. 3,949,206, issued Apr. 6, 1976 to Edwards et al U.S. Pat. No. 4,120,035 issued Oct. 10, 1978 to Cases et al, U.S. Pat. No. 4,313,195, issued Jan. 6, 1982 to Lehmann, and U.S. Pat. No. 4,322,810, issued Mar. 3, 1982 to Nakayama.
Until the present invention, microprocessors have not been used in systems to provide filtering where the system data rates are high, in the order of several tens of microseconds, since known algorithms used to form a digital filter requires one or more multiplications, each requiring milliseconds of computational time, during which the computer cannot be used to process other data.
Also, a CFAR (Constant False Alarm Rate) scheme for detecting the presence of a sine wave in Gaussian noise, known as the Tricky system, is known to the art. In this system, the received signal is wideband filtered, and is then hard limited to provide a constant amplitude signal having either a first or a second instantaneous value. This hard limited signal is then filtered through a narrow band analog filter, and integrated to provide the desired output signal. This system is described in detail in a University of Florida report #0007-8, dated June 28, 1968, by Watterson, Dickson, and Johnson, entitled "The Tricky System: A CFAR scheme for detecting the presence of a sine wave in Gaussian noise".